FIELD OF THE INVENTION
The present invention relates to the field of waveform generators for
producing waveforms for operating CRT displays and, more particularly, to a
waveform generator for producing a horizontal or vertical dynamic focusing voltage
waveform.
BACKGROUND OF THE INVENTION
Devices as disclosed in Japanese Laid-open Patents S61-151591, H1-191895,
and H4-114589 are well-known waveform generators for applying dynamic
focusing voltage to CRTs.
Fig. 6 is a block diagram of the major parts of a waveform generator
(dynamic focusing circuit) as disclosed in Japanese Laid-open Patent H1-191895 as
an example of the waveform generator of the prior art. The prior art comprises a VCO
(voltage control oscillator) 61, a binary counter 62, D/A (digital to analog) converter
63, multiplication unit 64, and amplifier 65.
Operation of the prior art is briefly explained with reference to Fig. 6.
The conventional waveform generator employs the clock pulse from the VCO 61 to
make the binary counter 62 count synchronizing to the scanning frequency. The D/A
converter 63 receives the output of the binary counter 62 for producing a saw tooth
wave.
Then, the multiplication unit 64 multiplies the signal, after removing the
direct current component from the saw tooth wave, by its inverted signal to produce a
parabolic waveform. The signal comprising a parabolic waveform produced by the
waveform generator is output as the dynamic focusing voltage.
As described above, the waveform generator of the prior art, in general,
employs a multiplication unit for converting a saw tooth wave synchronized to the
scanning frequency to a signal comprising a parabolic waveform. The parabolic
waveform signal which has its minimum value at the center is then amplified to
generate a dynamic focusing voltage waveform.
With increasing flatness of a CRT display screen, the optimal dynamic
focusing voltage waveform is tending towards being in proportion to the distance
from the screen center raised for example to the 2.8th power, whereas a parabolic
waveform, which has its minimum value at the center, is proportional to the square of
the distance from the screen center.
Therefore, the waveform generator of the prior art which generally
produces a parabolic waveform is becoming unsuitable for producing the optimal
dynamic focusing voltage waveform for more recent, flat-screen CRTs. The prior art
may fail to achieve the optimal focus characteristics over the entire screen. This is a
first disadvantage of the prior art.
The output waveform produced by a waveform generator is usually
several volts, but CRTs require several hundreds of volts as the dynamic focusing
voltage waveform.
Accordingly, the signal produced by the waveform generator needs to
be amplified for use by CRTs. To amplify the voltage at low cost, the focusing circuit
of the prior art employs a transformer to increase the voltage and supply the increased
dynamic focusing voltage waveform to the CRT.
The use of transformers, however, narrows the range of optimal
frequency and phase characteristics of the focusing circuit. For example, if the
focusing circuit employs a transformer which is satisfactory around the horizontal
frequency of 100 kHz for amplifying the horizontal frequency around 30 kHz, the
actual dynamic focusing voltage may become asymmetric even though the waveform
generator outputs a symmetric waveform. Comparing the left (L) and right (R) from
the center (C), as shown in Fig. 5, the actual dynamic focusing voltage has distorted
asymmetric waveform.
Therefore the waveform generator of the prior art may produce a
distorted dynamic focusing voltage waveform for some horizontal frequencies when
it is required to process a broad range of horizontal frequencies such as the case with
the latest CRT display monitors for computers.
The prior art may have difficulty in assuring the optimal dynamic
focusing characteristics for the entire range of horizontal frequencies. This is the
second disadvantage of the prior art.
SUMMARY OF THE INVENTION
The present invention employs a conversion unit which produces a
dynamic focusing voltage waveform for CRTs.
The conversion unit converts a position signal corresponding to the
position on the CRT screen to a converted signal which may be optimal dynamic
focusing voltage for CRTs.
A waveform of the converted signal may require modification to correct
deterioration or deformation of the waveform caused by the dynamic focusing output
circuit applied used over a broad synchronizing frequency range.
For this purpose, the present invention employs a correction unit for
selecting an appropriate constant for the conversion unit and correction unit itself, and
thereby correcting the converted waveform, which is made by converting the position
signal, to make a corrected position signal.
The corrected position signal is reconverted using the same conversion
unit to produce the expected dynamic focusing voltage waveform. Thus, the
waveform generator of the present invention enables the supply of an optimal
dynamic focusing voltage waveform over a broad synchronizing frequency range.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1(a) is a block diagram of a waveform generator in accordance with
a first exemplary embodiment of the present invention.
Fig. 1(b) is a block diagram of a waveform generator in accordance with
a second exemplary embodiment of the present invention.
Fig. 2 is a block diagram of a waveform generator in accordance with a
third exemplary embodiment of the present invention.
Fig. 3 is a graph illustrating an example of characteristics of a
conversion unit 11.
Fig. 4 is a graph illustrating exemplary waveforms.
Fig. 5 is an example of a focusing output with unfavorable frequency
and phase characteristics.
Fig. 6 is a block diagram of a waveform generator in accordance with
the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First exemplary embodiment
Fig. 1(a) is a block diagram of a waveform generator in accordance with
a first exemplary embodiment of the present invention.
The waveform generator of the present invention uses a position signal
value px, offset coefficient -B, a coefficient F for correcting the scanning frequency,
and an amplifying coefficient A for generating the dynamic focusing voltage
waveform output. The present invention first prepares a saw tooth wave signal
corresponding to the position on the screen for producing a dynamic focusing voltage
waveform. This is hereafter called the position signal px.
Configuration of the first exemplary embodiment is explained with
reference to Fig. 1(a). The position signal px is input to a conversion unit 11 which
generates a specified exponential function, for example px2.8. A first addition unit 12
adds the output of the conversion unit 11 and the offset coefficient -B. A first
multiplication unit 13 multiplies the output of the first addition unit 12 by the scanning
frequency correcting coefficient F. A second addition unit 14 adds the output of the
first multiplication unit 13 and the position signal px. The conversion unit 11 again
receives the output of the second addition unit 14. A second multiplication unit 15
then multiplies the output of the conversion unit 11 by the amplitude coefficient A.
The addition unit 12, first multiplication unit 13, and second addition unit 14 form a
correction unit 17.
Switches 16A and 16B are provided to inter-switch the input and output
of the conversion unit 11. When the position signal px is input to the conversion unit
11, the first addition unit 12 receives the output of the conversion unit 11. When the
output of the second addition unit 14 is input to the conversion unit 11, the second
multiplication unit 15 receives the output of the conversion unit 11.
The position signal px is obtainable from a saw tooth wave produced by
a digital binary counter, as in the prior art. It can alternatively be obtained by
converting the waveform of a saw tooth wave, which linearly decrease or increase
during CRT scanning, to a digital signal using an A/D converter.
The position signal corresponds to the scanning position (or data point)
x on the screen. The position signal has the minimum value -P at the starting edge on
the screen, 0 at the screen center, and the maximum value +P at the ending edge of the
screen.
The scanning position (or data point) x takes a value -n at the starting
edge of the screen, 0 at the screen center, and n at the ending edge of the screen. In
other words, the center value of the saw tooth wave corresponds to the screen center.
The position signal at each data point x (-n ......-1, 0, 1, ........n) is set as
px.
The position signal px is input to the conversion unit 11, and the
conversion unit 11 outputs a corresponding functional value f(px) to the first addition
unit 12. The px is considered to be a normalized value because the value of the
position signal px later becomes the basis for correction and amplification before
output.
To support f(0) = 0, f (-P) = f(+P), the input signal of the conversion unit
is converted to an absolute value at an input unit of the conversion unit 11 so that the
conversion unit 11 always conducts operation on a positive input value.
The conversion unit 11 comprises a memory for storing function values.
This memory desirably employs the input signal as a variable and stores values in a
part of the first quadrant near the origin of settable exponential function and those
mirrored to the second quadrant symmetrical to the axis.
In other cases the conversion unit comprises an absolute value circuit
for converting the input value to an absolute value and a memory for storing that
function values in a part of the first quadrant near the origin of a specified exponential
function which employs the input signal converted to an absolute value as a variable.
The first addition unit 12 and the second addition unit 14 can be either a
digital adder or an analog processor having a D/A converter unit.
In the same way, a first multiplication unit 13 and a second
multiplication unit 15 can be a digital multiplier or an analog multiplier, if the addition
unit is configured with an analog circuit. Likewise, addition unit and multiplication
unit can be configured in required combinations.
Table 1 is an example of the
conversion unit 11 having the exponent 2.8
when P=253 and f(±P) = 63. Fig. 3 is a graph illustrating the values in Table 1.
px | f(px) | px | f(px) | px | f(px) | px | f(px) |
0 | 0 | 155 | 16 | 199 | 32 | 230 | 48 |
58 | 1 | 158 | 17 | 201 | 33 | 231 | 49 |
74 | 2 | 162 | 18 | 203 | 34 | 233 | 50 |
85 | 3 | 165 | 19 | 205 | 35 | 235 | 51 |
95 | 4 | 168 | 20 | 207 | 36 | 236 | 52 |
102 | 5 | 171 | 21 | 209 | 37 | 238 | 53 |
109 | 6 | 174 | 22 | 211 | 38 | 239 | 54 |
115 | 7 | 177 | 23 | 213 | 39 | 241 | 55 |
121 | 8 | 179 | 24 | 215 | 40 | 243 | 56 |
126 | 9 | 182 | 25 | 217 | 41 | 244 | 57 |
131 | 10 | 184 | 26 | 219 | 42 | 246 | 58 |
136 | 11 | 187 | 27 | 221 | 43 | 247 | 59 |
140 | 12 | 189 | 28 | 223 | 44 | 249 | 60 |
144 | 13 | 192 | 29 | 224 | 45 | 250 | 61 |
148 | 14 | 194 | 30 | 226 | 46 | 252 | 62 |
152 | 15 | 196 | 31 | 228 | 47 | 253 | 63 |
The offset coefficient -B is pre-input to the first addition unit 12. The
output of the first addition unit 12 is
f (px) -B
This is input to the multiplication unit 13. The coefficient F is pre-input
to the multiplication unit 13 as the scanning frequency correction coefficient. The
results of multiplication by the multiplication unit 13 is
F {f (px) -B}
This is input to the second addition unit 14.
The aforementioned normalized value px is also input to the second
addition unit 14. The output of the second addition unit 14 is
px + F {f (px) -B}
This formula indicates that a position correction value
F {f (px) -B}
is added to the original normalized position signal px. If B = f (±P), the
position correction value
F {f (px) -B}
becomes 0 at the starting and ending edges of the screen, and the
minimum value at the screen center. In other words, it can be understood that the
largest delay in the phase occurs at the screen center compared to the original position
signal px. This is defined as a corrected position signal:
p'x = px + F {f (px) -B}
The corrected position signal p'x is input to the
conversion unit 11
again, and the
conversion unit 11 outputs the function value f (p'x). This is input to the
multiplication unit 15. Table 2 shows an example of details of operation up to this
point.
An example of conversion and calculation
(When B = 63 and F = 0.77) |
x | px | abs(px) | f(abs(px)) | f(px)-B | F(f(px)-B | p'x | abs(p'x) | f(abs(p'x)) |
1 | -255 | 255 | 64 | 1 | 1 | -254 | 254 | 64 |
2 | -220 | 220 | 43 | -20 | -16 | -236 | 236 | 52 |
3 | -185 | 185 | 26 | -37 | -28 | -213 | 213 | 39 |
4 | -150 | 150 | 15 | -48 | -37 | -187 | 187 | 27 |
5 | -115 | 115 | 7 | -56 | -43 | -158 | 158 | 17 |
6 | -80 | 80 | 3 | -60 | -47 | -127 | 127 | 9 |
7 | -45 | 45 | 1 | -62 | -48 | -93 | 93 | 4 |
8 | -10 | 10 | 0 | -63 | -49 | -59 | 59 | 1 |
9 | 25 | 25 | 0 | -63 | -48 | -23 | 23 | 0 |
10 | 60 | 60 | 1 | -62 | -48 | 12 | 12 | 0 |
11 | 95 | 95 | 4 | -59 | -45 | 50 | 50 | 1 |
12 | 130 | 130 | 10 | -53 | -41 | 89 | 89 | 3 |
13 | 165 | 165 | 19 | -44 | -34 | 131 | 131 | 10 |
14 | 200 | 200 | 33 | -30 | -23 | 177 | 177 | 23 |
15 | 235 | 235 | 51 | -12 | -9 | 226 | 226 | 46 |
16 | 255 | 255 | 64 | 1 | 1 | 256 | 256 | 65 |
16 | 270 | 270 | 76 | 13 | 10 | 280 | 280 | 83 |
The constant A is pre-input to the multiplication unit 15 as an amplitude
coefficient, and the multiplication unit 15 outputs the result of multiplication: Af
(p'x). This is the dynamic focusing output:
W = Af (p'x) = Af [px + F {f(px) -B}]
If the scanning frequency correction coefficient F is 0, W = Af (px).
This is merely the amplitude of a specified exponential waveform set in the
conversion unit 11 multiplied by the amplitude coefficient A.
Therefore, the first exemplary embodiment of the present invention
solves the first disadvantage of the waveform generator of the prior art previously
described. More specifically, the first exemplary embodiment enables the generation
of an ideal dynamic focusing voltage waveform for increasingly flat CRT display
monitors for which compensation using a parabolic waveform has become difficult.
When the scanning frequency correction coefficient F increases, the
dynamic focusing output W deforms, generating the largest delay in the phase at the
screen center due to the effect of the corrected position signal p'x.
Thus, the first exemplary embodiment of the present invention solves
the second disadvantage of the waveform generator of the prior art previously
described by controlling the scanning frequency correction coefficient F. In other
words, the distortion of a waveform which is a problem caused by the employment of
a low-cost transformer in the prior art can be corrected optimally.
As explained above, the waveform generator of the present invention
solves the first disadvantage of the prior art by employing the function in which the
exponent can be specified for producing the ideal dynamic focusing voltage
waveform required for recent CRT display monitors which are becoming difficult to
satisfy using a parabolic waveform.
Furthermore, the waveform generator of the present invention is
capable of optimizing correction by selecting an appropriate constant: A, B, or F for
processors, which solves the second disadvantage of the prior art: that is, distortion of
waveform caused by the dynamic focusing output circuit which has deficient
frequency and phase characteristics.
Fig. 4 shows an example of changes in the dynamic focusing output
waveform when the value F is changed. When F = 0, the waveform is inclined towards
the left. On the other hand, when F = 0.77, the waveform is inclined to the right. The
waveform generator of the present invention thus enables the cancellation of any
distortion generated by the dynamic focusing output circuit by generating signals with
such waveform. Furthermore, a waveform which is desirably bisymmetrical is
obtained. As shown, it is preferred that the curve minimum coincide with the x-axis.
Second exemplary embodiment
The coefficients A, B and C can be pre-input to the first addition unit
12, the first and the second multiplication unit 13, 15 using hard-wiring or reading
them stored in a memory. The coefficients A, B and C can be determined by picture
quality determination experiments changing values of the coefficients.
In the above explanation, the input and output of single conversion unit
11 are switched. However, it is also possible to provide two conversion unit 11A and
11B with the same function (or two different functions) as in Fig. 1B.
Third exemplary embodiment
A third exemplary embodiment of the present invention is explained
with reference to a block diagram of Fig. 2.
The first and second exemplary embodiments were explained referring
to a hardware circuit configuration. The third exemplary embodiment may be
implemented, for example, using a CPU.
In the waveform generator according to the second exemplary
embodiment of the present invention, a ROM 21 for storing function data, a RAM 23
for storing waveform data, and a CPU 24 for processing are connected to a bus 20.
The RAM 23 receives the output of a counter 22 which is initialized by the
synchronizing signal S. A D/A converter 25 receives the output of the RAM 23.
The CPU 24 calculates the focusing voltage waveform data
corresponding to the position on the screen using the function data previously stored
in the ROM 21, and the result thereof is stored in the RAM 23.
The counter 22 reads out the waveform data corresponding to the
position on the screen from the RAM 23, and causes to output the dynamic focusing
voltage waveform.
An offset coefficient -B, scanning frequency correction coefficient F,
and amplitude coefficient A are stored in the internal register of the CPU 24, ROM 21
or RAM 23 illustrated, or other memory not illustrated in the figure. Needless to say,
the CPU 24 may conduct more than one addition and multiplication processing.
The functions ROM 21, RAM 23, and CPU 24 may be combined in
alternative arrangements. For example, the function data can be stored in a part of
fixed program area in the CPU instead of using the ROM 21. The RAM can be
substituted with a part of the main storage in the CPU. The counter 22 can also be
replaced with a DMA controller. The equivalent function can also be realized by using
a part of the main storage as the RAM 21 and RAM 23, and function data is
transferred by processing of the CPU 24.
In Fig. 2, a so-called refresh memory is configured by continuously
reading out data in a specified storage area of the RAM 23 by the counter 22. The
synchronizing signal S initializes the counter 22 for keeping the readout timing. The
CPU 24 sets the dynamic focusing voltage waveform data to the specified storage area
of the RAM 23.
More specifically, the CPU 24 first produces a position signal px
corresponding to the scanning position x on the screen, which has the minimum value
-P at the stating edge of the screen, 0 at the screen center, and the maximum value +P
at the ending edge of the screen.
Next, the CPU 24 reads out the function data f (px) stored in the ROM
21 based on the position signal px. The CPU 24 then calculates the corrected position
signal p'x = F {f (px) -B} using the offset coefficient -B and the scanning frequency
correction coefficient F. Consequently, the original normalized position signal px and
the position correction value F {f(px) -B} are added to obtain p'x = px + F{f(px) -B}.
As already explained in the first exemplary embodiment, it can be understood that the
corrected position signal p'x causes the largest phase delay at the screen center
compared to the original position signal px.
Then, the CPU 24 reads out the function data f(p'x) stored in the ROM
21 based on the corrected position signal p'x, and calculates the dynamic focusing
output W = Af(p'x) = Af[px+F{f(px) -B}] using the amplitude coefficient A.
The CPU 24 then writes a calculated value to the RAM 23.
By repeating this operation, a series of dynamic focusing output data is
completed in the specified area of the RAM 23.
As explained above, the waveform generator of the present invention
employs the function in which the exponent can be specified for solving the first
disadvantage of the prior art. More specifically, the present invention enables the
generation of an ideal dynamic focusing voltage waveform for the latest CRT display
monitors which are becoming difficult to fully compensate using a parabolic
waveform.
Furthermore, in the waveform generator of the present invention,
appropriate constants A, -B, and F for processors are selectable as required for solving
the second disadvantage of the prior art. More specifically, distortion caused by the
dynamic focusing output circuit which has deficient frequency and phase
characteristics can be corrected optimally by selecting appropriate constants A, -B,
and F.
Fig. 4 is an example of change in the dynamic focusing output
waveform when a value of the constant F is changed. When F = 0, the waveform is
inclined towards the left, and when F = 0.77, the waveform is inclined towards the
right. The waveform generator of the present invention thus enables the cancellation
of any distortion caused by the dynamic focusing output circuit by generating signals
with such waveform.
The third exemplary embodiment of the present invention can solve the
first and second disadvantages of the prior art as explained in the first exemplary
embodiment. In addition, the waveform generator of the third exemplary embodiment
can be realized by the use of a part or all of the CPU, ROM, and RAM for controlling
the entire CRT display monitor. Thus, the third exemplary embodiment offers a low-cost
CRT display monitor with optimal focusing performance.
Accordingly, the present invention solves a problem of a waveform
generator which produces a dynamic focusing waveform not suitable for CRTs
requiring non-parabolic waveform, and cancels distorted frequency and phase
characteristics of the output circuit. The present invention realizes a waveform
generator which assures high-quality dynamic focusing characteristics.
In the exemplary embodiments, an example of the employment of
exponential function is explained in detail. The type of function is naturally selected
in accordance with the characteristics of CRT displays. It will be recognized that other
types of functions may be employed in accordance with the present invention. The
exemplary embodiments are also explained with the precondition that the dynamic
focusing voltage waveform is applied to the CRT in the horizontal deflecting
direction. It will be appreciated that the same effect is achieved by applying the
waveform in the vertical direction. The exemplary embodiments are also explained
with the precondition of the use of a digital signal and digital circuit. It will also be
appreciated that the present invention can be realized with the use of an analog signal
and an analog circuit.
The exemplary embodiments described herein are therefore illustrative
and not restrictive. The scope of the invention being indicated by the appended claims
and all modifications which come within the true spirit of the claims are intended to be
embraced therein.
Reference numerals for drawings
- 11
- Conversion unit
- 12
- First addition unit
- 13
- First multiplication unit
- 14
- Second addition unit
- 15
- Second multiplication unit
- 16A, 16B
- Switch
- 20
- Bus
- 21
- ROM
- 22
- Counter
- 23
- RAM
- 24
- CPU
- 25
- D/A converter